Machine readable matrix symbols are a special case of optically scannable symbols which often use circles or square based cells instead of variable widths of spaces and lines, such are used in binary bar codes, to represent the data.
Historically, the automatic identification industry has relied on optical cameras for reading matrix symbols. Optical cameras possess good performance when reading matrix symbols printed on papers or labels, but may have problems when reading matrix symbols that are directly marked on substrates using marking methods such as dot-peen or laser Moreover, when direct marked matrix symbols are painted, the challenge of reading the symbols using optical contrast readers becomes even larger.
Ultrasound as a means of reading matrix symbols is possible if that backscattered ultrasound signal corresponding to the matrix symbol differs from that corresponding to the substrate where the matrix symbol is comprised. This is achieved if the matrix symbol is applied to the substrate using materials with different acoustic impedance properties than the acoustic impedance properties of the substrate, or if the ultrasound propagation time to the matrix symbol is different than to the substrate.
Using ultrasound as a means for reading matrix symbols has been suggested in U.S. Pat. No. 5,733,811. They proposed an apparatus for reading matrix symbols using an ultrasound transducer which is physically scanned by moving it over the component bearing the two-dimensional symbol in a raster pattern.